The present invention relates to optical recording systems. In particular, the present invention relates to systems for positioning the optical apparatus which follows a particular data track on the optical recording media in a desired position on a particular track of the optical recording media and for moving that optical apparatus from one track position to another. This invention is not, however, limited in scope to optical recording and may also find application in high density magnetic recording and other applications where a large area must be accessed with precision in a short time interval.
Prior art systems for positioning optical apparatus in optical recording media typically depends upon fine movements of an optical lens system in dimensions of a comparatively few track spaces with respect to the media by a first system. The first system is in turn mounted and carried by some type of carriage assembly or second system for comparatively large track movements with respect to recording media so that the entire recording media is accessed. It is necessary to control both movement of the first system and of the second system. In some prior art systems the first system is used for fine tracking of a particular track on the recording media in a track following mode when a track is being accessed. The large movement or coarse track accessing system is used for movements from one track to another. The fine tracking system is controlled in the prior art by an optically actuated servo loop mechanism which develops servo positioned error signals directly from optical signals received from the media. In such prior art systems, the coarse actuator system is controlled by track accessing signals developed from the present track address location and a differential signal developed with respect to the desired track location. Such signals are sometimes used in association with a fixed position indicator reticle which provides a position reference indication of the coarse actuator system with respect to the fixed framework of the optical recording system. Such signals may be comparatively inaccurate compared to the very high track densities achieved in optical recording systems and compared with the speed and accuracy with which it is necessary to move from one track to another track in accessing data in optical recording systems.
Therefore, there is a need for an improved optical recording track following and track seeking control system to quickly and reliably cause a controlled movement of the optical apparatus from a first position to a second position.
U.S. Pat. No. 3,969,574 issued July 13, 1976 shows an optical recording system in which position information for the optical elements is developed from a reference signal between the moving actuator mechanism and the stationary framework or chassis assembly of the optical apparatus.
United Kingdom Patent Application No. 2,029,051, published Mar. 12, 1980, shows a single servo loop system for controlling the positioning of the optical apparatus in optical recording together with a second servo control loop based on a position indicator which senses the radial position of the optical apparatus with respect to the optical recording media should the first servo control loop fail. The primary servo control loop maintains track position based on a combination of track addressing information and track following information provided optically from the recording media.
U.S. Pat. No. 4,034,403 shows an optical recording system in which an optical positioning control system develops both a coarse positioning signal and a fine positioning signal using the same optical information derived from the optical recording media. Optical gratings are used to develop the information signals.
None of the foregoing systems relating to optical recording show or anticipate a system according to the present invention where a control system for head positioning in an optical recording system is controlled by two servo control loops where the first control loop receives track following and track seeking servo control information and where the second servo control loop controls coarse positioning of the optical system solely by position sensing signals of the relative position of the second system to that of the first system.
In the magnetic recording control system art, there are servo control systems known for use with high-density track following requirements. Because the optical recording art also relates to high-density track following systems and because other elements of the technology may be considered pertinent, the relevant prior art also includes such control systems. For example, U.S. Pat. No. 3,924,268 shows a control system for positioning a magnetic disk transducer head including a primary carriage for coarse positioning and a secondary low-mass carriage riding on the first carriage in order to position a magnetic recording head relative to a magnetic recording medium. By supporting a secondary low-mass carriage on a primary coarse positioning carriage, the patent shows that highly accurate extremely fast response can be achieved over small distances. However, the patent shows that the control system works by simultaneously and continuously controlling both carriages in response to a single position error control signal with a secondary carriage being biased toward a zero displacement position relative to the primary carriage. The system shown in the patent uses position detection and velocity detection in order to provide servo control signals for the primary and secondary actuators. The patent does not teach the use of a control system for controlling the coarse positioning actuator solely by means of the relative position of the secondary or fine actuator with respect to that of the coarse position or primary actuator as does the present invention. Therefore, this patent does not show or anticipate the teachings of the present invention.
U.S. Pat. No. 3,725,764 shows position and velocity detection for controlling a servo motor mechanism in order to follow a servo track on a moving surface of a magnetic disk memory. This patent does not show or anticipate the present invention but is cited to show the state of the prior art.